1. Field of the Invention
The present invention relates to a thread-wound golf ball of a multilayer structure having a rubber thread layer wrapped around the center, exhibiting favorable approach spin performance and shot feeling, great flight distance, and superior in scuff resistance.
2. Description of the Background Art
A thread-wound golf ball has a structure in which a rubber thread layer formed by winding a tensioned rubber thread around a solid center or a liquid center is surrounded entirely with a cover. The thread-wound golf ball generally having the favorable shot feeling and superior approach spin performance is disadvantageous in that the spin rate is too high and the launch angle is small. Therefore, there was a problem that the flight distance cannot be easily increased. Thus, amateur golf players generally have a tendency for using a solid golf ball exhibiting long flight distance than such a thread-wound golf ball.
Several approaches have been made to increase the flight distance of a conventional thread-wound golf ball with a solid center. For example, U.S. Pat. No. 5,848,942 discloses the definition of the center diameter to 30-38 mm, and the center hardness distribution, the cover flexural modulus and the like to a predetermined range. U.S. Pat. No. 5,853,337 discloses the approach of using a cover of high hardness as well as defining the center diameter to 30-38 mm and defining the difference between the load deformation of the center and the load deformation of the ball. Japanese Patent Laying-Open No. 10-201881 discloses the definition of the load deformation of the center, the cover hardness, and the flexural modulus to a predetermined range with the center diameter set to 30-38 mm.
Japanese Patent Laying-Open No. 9-294830 discloses a thread-wound golf ball with a non-yellowing thermoplastic polyurethane elastomer as the main component for the cover, wherein the specific gravity difference between the center and cover is not more than 0.2.
However, a thread-wound golf ball exhibiting sufficient great flight distance while maintaining a superior approach spin performance and shot feeling is not yet available even by the techniques disclosed in U.S. Pat. No. 5,848,942, U.S. Pat. No. 5,853,337 and Japanese Patent Laying-Open No. 10-201881. It is to be noted that ionomer resin is used as the main material for the cover of such thread-wound golf balls. Therefore, the shot feeling was hard. Also, the surface of the cover is easily scratched when hit with a short iron. The surface of golf ball is napped or scuffed, exhibiting poor scuff resistance.
The thread-wound golf ball of Japanese Patent Laying-Open No. 9-294830 employing thermoplastic polyurethane elastomer for the cover material could not achieve both favorable approach spin performance and great flight distance since the volume ratio of the rubber thread layer was not taken into account.
In view of the foregoing, an object of the present invention is to provide a thread-wound golf ball exhibiting favorable approach spin performance and shot feeling with a flight distance equal to or greater than the flight distance by a solid golf ball, and superior scuff resistance.
The inventors endeavored to achieve the above object. The inventors found that a thread-wound golf ball exhibiting a flight distance greater than that of a solid golf ball while maintaining favorable approach spin performance and shot feeling can be obtained by optimizing the balance between the volume ratio of the rubber thread layer to the thread-wound golf ball and the tension of the rubber thread wound around the center. The inventors also found that the scuff resistance can be improved by employing a cover with thermoplastic polyurethane based elastomer as the main material.
The thread-wound golf ball of the present invention has a cover wrapped around a rubber thread layer formed by winding a rubber thread around the center. The ratio of the volume of the rubber thread layer to the volume of the entire thread-wound golf ball is 10-20%. When deformation formed by applying an initial load of 98N to a final load of 1274N to the center is A, and deformation formed by applying an initial load of 98N to a final load of 1274N to a thread-wound core having a rubber thread wound around the center is B, the difference in deformation (Axe2x88x92B) is 0.5-0.7 mm. The cover includes thermoplastic polyurethane based elastomer as the main material. The shore D hardness of the cover is 40-55.
In the thread-wound golf ball of the present invention, it is preferable that the thickness of the cover is 0.8-1.8 mm, deformation A formed by applying an initial load of 98N to a final load of 1274N to the center is 3.3-3.9 mm, and deformation B formed by applying an initial load of 98N to a final load of 1274N to the thread-wound core is 2.7-3.4 mm.
In the thread-wound golf ball of the present invention, the di-isocyanate constituting the thermoplastic polyurethane based elastomer included as the main material in the cover is preferably alicyclic di-isocyanate.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawing.